Apple last week gave six top executives $12.1 million each in stock grants that will vest over the next four years, according to filings with the U.S. Securities and Exchange Commission (SEC).

Apple last week gave six top executives $12.1 million each in stock grants that will vest over the next four years, according to filings with the U.S. Securities and Exchange Commission (SEC).

Those filings did not spell out why the grants were given, but they came hard on the heels of Peter Oppenheimer's March 4 retirement announcement, so they may have been part of Apple's overall executive retention plan. Oppenheimer is currently chief financial officer, but will step down at the end of September.

Oppenheimer will be replaced by Luca Maestri, Apple's vice president of finance and corporate controller, who joined the Cupertino, Calif. company in March 2013 after years with General Motors, Nokia and Xerox.

The awards of 22,738 "restricted stock units," or RSUs, were given to Eddy Cue, who heads Apple's Internet software and services group; Craig Federighi, the lead for the company's software engineer teams; Daniel Riccio, senior vice president of hardware engineering; Philip Schiller, head of marketing; Bruce Sewell, Apple's general counsel; and Jeffrey Williams, the top operations executive.

Of Apple's top leadership team, only Jonathan Ive, head of design, did not receive an RSU grant. The RSUs were handed out on Tuesday, March 4, and the pertinent documents were filed with the SEC on Thursday, March 6.

According to the SEC filings, the RSU grants will vest in thirds on April 1 of 2016, 2017 and 2018, assuming the executive is still with Apple.

At Monday's closing price, the shares were worth $12,072,059 to each of the six executives. RSUs are not actually awarded at the time of the grant, but instead are given when they're vested. The shares will be worth their then-current price, which could, of course, be dramatically different than their value Monday.

In the same filings, Apple also reported that it had assigned each of the six executives another 13,042 RSUs that will vest in two chunks on Oct. 1, 2015, and Oct. 1, 2016.

Those grants, however, were listed as "targets" that could end up twice the number of RSUs, or be reduced to zero, depending on Apple stock performance between September 2013 and the same month in 2016.

How much the executives receive will be based on "total shareholder return" (TSR), a combination of share price appreciation and dividends. If Apple outperforms the S&P 500 in TSR, they will presumably receive all -- or up to 200% of the target -- of the RSUs scheduled to vest that year. If Apple's TSR compares poorly, the RSUs will be reduced.

Apple did not spell out the TSR metrics in its filings, however.

Some of CEO Tim Cook's 1 million RSU award from the fall of 2011 are also at risk in a similar TSR-based calculation. When Apple announced in June 2013 that Cook had requested some of his RSUs be tied to stock performance, the company noted that the CEO was "leading by example" in a new push to link performance with stock awards given to the company's leadership team.

Last week's awards, and the fact that at least some were pegged to TSR, were the culmination of that push.

Because of the lack of detail about the vesting criteria, it was impossible to pin down how much each of the six executives could receive in 2015 and 2016. If all the shares vest, they would be worth just over $7 million at Monday's closing price, a figure that could as much as double -- to $14.1 million -- for each executive.